1. Field of the Invention
The present invention relates to sprockets and gears made from sintered metals which are produced by powder metallurgy and production method for the same, and specifically relates to improvements for wear resistance.
2. Description of the Related Art
This kind of sintered gears are largely made from Fe-based alloys, which have densities of about 6.8 to 7.2 Mg/m3 after compacting a powder and sintering the compact. In sprockets and heavy duty gears, wear resistance of a tooth flank is greatly required. In order to meet the requirement, it is effective to heighten density of sintered alloys. In order to enhance the density of Fe-based sintered alloys, hot forging may be effective. Although hot forging may allow producing gears with high density in which there are approximately no pores, the gears may have superfluous quality when the gears are not required to have entirely high density. Furthermore, weight of the gears is increased and advantages of porous body such as damping capacity and oil impregnation may be lost. Hot forging requires an apparatus to compress a work at high temperature and a specific means to avoid from oxidizing of the work while it is heated. Therefore, hot forging is complicated and the production cost is high.
Japan Patent Unexamined Publication No. 2003-253372 proposes cold forging to sintered bodies for another method to enhance the density. In the method of the reference, an Fe-based metal powder produced by partially diffusing 1 mass % of Mo particles to an iron powder containing 0.15 mass % of Mn and 0.3 mass % of a graphite powder are mixed, and the mixed powder is compacted to have a density less than 7.3 Mg/m3. The compact is sintered at a temperature of 950 to 1300° C., and is then forged in a closed or sealed die. In this method, forged parts with densities of 7.35 to 7.45 Mg/m3 when the forging pressure was 784 MPa is yielded, and forged parts with densities of 7.52 to 7.65 Mg/m3 when the forging pressure was 1177 MPa is yielded.
Japan Patent Publication No. 48-33137 proposes rolled gears for another method to enhance the density of sintered gears. In rolling of sintered bodies, inner pores are not changed after rolling, and only the surface layer of the tooth flank is rolled and densified, whereby pitching wear resistance is improved and gear accuracy is improved in the sintered gears.
The cold forging proposed in Japan Patent Unexamined Publication No. 2003-253372 has an advantage in which gears are densified by forging at room temperature. However, since a tablet-shaped sintered body is compressed and outwardly expanded in a die, thereby closely contacting the body with the die so as to form a tooth profile, high compression pressure is required. Therefore, the density of the end portion of the tooth is readily lowered when the tooth length is long. Furthermore, the weights of the sintered bodies are not uniform and there may be cases in which the weight exceeds the predetermined value. Therefore, when the sintered body is compressed to have the true density, there may be cases in which the volume after compression exceeds the predetermined value which is identical to the volume of the cavity of the die, whereby the die is broken. In order to avoid such an accident, compression may be performed using a die in which the excess volume of the material flows out the die and forms a flange as similarly as in the case of hot forging. In this case, a process for removing the flange is required and the number of processes is increased.
On the other hand, the rolling to sintered gears proposed in Japan Patent Publication No. 48-33137 has advantages in which the a tooth flank and a tooth bottom land are densified and high size accuracy can be obtained, and wear resistance can be improved. However, the method has a disadvantage in which the rolling needs long time. Furthermore, in gears with short module, rolling amount is not sufficient, whereby uniform densification is difficult. Moreover, the method is not suitable for densification of a tooth portion (entire tooth portion) and a shaft hole portion.
In order to solve the above problems, a method for forming a sintered gear was proposed. As shown in FIGS. 14 to 16, the thickness of a tooth portion is set shorter than other portion, and only the tooth portion is compressed and densified in the thickness direction in a recompression process. This method is similar to the above mentioned cold rolling, and has advantages in which since only the tooth portion is compressed, the pressure of a punch is reduced and the load exerted to a die is reduced. FIGS. 14A and 14B are a side view and a sectional view of a sprocket 231 to be produced. As shown in FIG. 15, in order to densify the tooth portion 235 of the sprocket 231, a sintered body 241 has a tooth forming portion 245 corresponding to the tooth portion 234, and an excess wall portion 245a has been formed at both ends of the tooth forming portion 245 in the thickness direction thereof. The tooth flank and the inner surface 242a of the shaft hole 242 are closely surrounded by a die 251 and a core rod 252. As shown in FIG. 16A, the excess wall portion 245a is compressed by an upper punch 253 and a lower punch 254. As a result, as shown in FIGS. 16B and 16C, the excess wall portion 245a is crushed and the tooth forming portion 245 is compressed, whereby the tooth portion 235 is densified.
However, in this method, as shown in FIG. 16C, not only the excess wall portion 245a is crushed to the compression direction (direction taken by allow A), but also the material is press out by plastic flow inwardly in the radial direction (direction taken by allow A′). As a result, although the material is compressed in the thickness direction, the material is flowed out inwardly in the radial direction, and there is a limit to densify the tooth portion 235. FIG. 16D shows an example of a density distribution of the sprocket 231 of which tooth portion 235 has been densified by this method. As shown in FIG. 16D, the density is gradually decreased from the densified tooth portion 235 to the inner circumferential portion 233 without the tooth portion 235 in a large area. This shows that amount of material flowed out from the tooth portion 235 to the inner circumferential portion 233 by plastic flow is large.